Field-effect transistors of the insulated-gate type (IGFET) have a high input impedance and are therefore particularly suitable for amplifying or otherwise processing incoming signals of the binary-coded type whose basic configuration must be preserved. However, the small capacitance existing between the insulated gate and the substrate of an IGFET is conducive to a breakdown of the intervening insulation even upon accumulation of minor charges on the gate. In order to prevent such a breakdown, it has become customary to combine such an IGFET with a Zener-type diode--formed in the same substrate--which is reverse-biased by the signal voltage (assumed to be unipolar in nature) but breaks down when that voltage exceeds a level that can be safely tolerated by the gate/substrate assembly.
As pointed out in U.S. Pat. No. 3,934,159, such a protective diode may become troublesome if the input voltage is not strictly unipolar but is overlain by parasitic oscillations which could bias the diode in its forward direction with resulting injection of minority carriers into the common substrate. The solution offered by that U.S. patent involves the provision of a second diode which is connected in parallel with the first one but does not share the substrate of the IGFET and which conducts in response to an inverted voltage before that voltage reaches the threshold at which minority carriers would begin to pass through the first diode.
The required physical isolation of the second diode from the common substrate of the first diode and of the IGFET protected thereby necessitates the use of different semiconductor bodies or the separation of the two diodes from each other by a cut in the substrate of such a body. The need for two distinct components in the first instance is a drawback where compactness is an important consideration; the additional operation of producing a gap is inconvenient in the second instance.